Parasitic nematodes are a major cause of human morbidity and contribute significantly to a loss of Disability Adjusted Life Years. More importantly, effective chemotherapy is still not available to control some infections, such as lymphatic filariasis or river blindness. Perhaps less well appreciated, but equally important for proper human nutrition and health, is the devastating economic impact of nematode parasites on livestock and plants and new anti-helminthic and drug targets are both desperately needed in all settings. Serotonin (5-HT) dramatically inhibits locomotion and the contraction of body wall muscle in a variety of free-living and parasitic nematodes, suggesting that serotonergic signaling may be an excellent target for drug discovery. We propose to use a dual systems approach, designed to exploit the experimental advantages of both the C. elegans and A. suum model systems, to characterize the receptors and downstream signaling components in the pathway of 5-HT inhibition of locomotion. First, the role of 5-HT signaling in the inhibition of locomotion will be characterized in C. elegans by bioinformatics, the heterologous expression of cloned receptors and the use of putative 5-HT receptor null mutants, with special focus on the identification of the receptors and their downstream signaling pathways. Second, potential orthologues of the relevant C. elegans 5-HT receptors will be cloned and characterized from A. suum and the proposed pathway of 5-HT inhibition of locomotion generated from C. elegans tested by examination of relevant physiological end points in A. suum (i.e. locomotion and muscle contraction). Because of the enormous diversity among nematodes, processes in C. elegans may not be duplicated exactly in the parasites, so the results of this study should not only identify key targets in serotonergic signaling pathways, but also highlight potential differences between these two important model systems. Parasitic nematodes cause significant medical, veterinary and agricultural problems worldwide. The present studies are designed to characterize the receptors and downstream signaling components involved in the 5-HT inhibition of locomotion. Locomotion is a key target for anti-nematodal drug discovery and these studies should provide basic insights into pathways regulating neurotransmitter release and the activity state of body wall muscle, as well as identifying a wealth of potential novel molecular targets for drug discovery. PROJECT NARRATIVE Parasitic nematodes cause significant medical, veterinary and agricultural problems worldwide. These studies are designed to characterize the receptors and downstream signaling components involved in serotonin inhibition of locomotion. Locomotion is a key target for anti-nematodal drug discovery and these studies should provide insight into pathways regulating neurotransmitter release and the activity state of body wall muscle, as well as identifying a wealth of potential novel drug targets.